Header set to NULL by a VMOD: semantic change between 3.0.3 and 3.0.6

[Geoff Simmons]

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Hello all,

A conversation on the IRC channel helped to clear up what went on here.

Short version for VCL authors: the way we should expect things to work
is as in the semantics of 3.0.6 (actually as of 3.0.4, apparently).

That is, if you're using a VMOD that may return NULL for a STRING
return type, and assign the return value to a header, then the header
will compare equal to "" if the VMOD returned NULL.

> set req.http.x-foo = vmod.foo(); if (req.http.x-foo == "") { # VMOD
> returned NULL (or empty string) }

This was the issue in ticket #1406:

https://www.varnish-cache.org/trac/ticket/1406

An earlier change that fixed an issue with string concatenation had
the side effect of changing the semantics of truthiness. The decision
in #1406 was to leave things as they are, since another change would
cause even more difficulty.

The change log (changes.rst) mentions this as happening between 3.0.2
and 3.0.3, but apparently the commit actually went into 3.0.4.


Happy VCL-ing!
Geoff
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